armv7m: remove magic numbers for number of core registers

Change-Id: I4296b812f0211011ccf3da8d203545dfba493903
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2053


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>